The present invention relates to a process for the non-destructive testing by eddy currents with a correction of the air gap effects and an apparatus for performing this process. It is used in the testing and inspection of metal parts, particularly tubes, plates, etc.
Inspection by eddy currents consists of studying the variations of the currents induced in a metal part by the magnetic field of a coil, traversed by an alternating exciting current. In turn these currents produce a magnetic field, which opposes the induction field, thereby modifying the impedence of the exciting coil. This coil is located in a sensor or probe moved along the member to be inspected or tested. Any fault or defect in the latter (change of size, variation in the conductivity, cracks, etc.) modifies the path or intensity of the eddy currents and correlatively the impedence of the coil.
The probe is generally constituted by two adjacent coils, supplied in opposition and placed in the two adjacent branches of a measuring bridge. The passage of a fault in the field of the probe unbalances the bridge twice, firstly in one direction and then in the other. The voltage supplied by the probe is analyzed and displayed on the screen of a cathode tube by its resistive component X (or real component), which is the component in phase with the exciting current, and its reactive component Y (or imaginary component) which is the component in phase quadrature with the exciting current. The complex voltage supplied by the probe is consequently represented by a point of coordinates X, Y. When a fault passes in the field of the probe, the representative point plots a curve having two lobes in the form of a figure of eight. Each fault can then be identified on the basis of the phase of the lobes (slope relative to the reference axis) and the amplitude thereof.
To obtain a good level of accuracy in the location of faults, it is necessary to use very small probes. However, the problem of air gap variations is quickly encountered. To provide a better understanding of this problem, which forms the basis for the invention, the question will firstly be defined in conjunction with FIGS. 1a and 1b.
FIGS. 1a and 1b illustrates two types of probe or sensor commonly used in the field of inspection and testing by eddy currents. These probes essentially comprise a body 2 provided with two windings Z.sub.1 and Z.sub.2. They travel in front of or in the member 4 to be inspected. The air gap is formed by the space separating the probe from the member to be inspected and is designated E in FIGS. 1a and 1b.
It is in fact an average air gap, because the probe is not necessarily strictly parallel to the member to be inspected. Therefore, there can be a "differential" air gap, designated e, which represents the difference between the air gaps of each winding. However, no account will be taken hereinafter of the differential air gap, whose effects can be corrected by filtering. The essence of the invention relates to the correction of the air gap E, being common to both windings.
If this air gap was constant throughout the measurement, it would only lead to an attenuation of the signal, which could be compensated by amplifying the detected signals. In actual fact, due to the movement of the probe along the member, said air gap varies constantly. Thus, there is a permanent and unpredictable modification of the measuring signal. Thus, the intensity of this signal not only depends on the detected fault, but also on the distance between the probe and the member to be inspected or tested. This is due to the fact that the intensity of the currents induced in an area of the member to be tested is dependent on the magnetic induction created in this area and that the latter decreases when the distance from the field windings increases.
In the same way, the action of the eddy currents on the windings is also dependent on this distance.
If it is wished to correct the effects of air gap variations on the result of the measurement, it is necessary to record the value of the air gap at all times. Even if such a process could be performed, it would be extremely difficult, because it would require not only air gap measuring means, but also correction circuits based on a law which, by its very nature, is very complex.